1. Field of the Invention
The present invention relates to a method of inspecting an electrically conductive object so as to detect the presence of an anomaly.
2. Description of Related Art
An anomaly can be a local decrease of the thickness of the object, which decrease is caused for example by corrosion.
European patent specification No. 321112 discloses a method of determining the thickness of a container wall means using a probe comprising a transmitter coil for inducing eddy currents in the object, and a receiver system for providing a signal indicative of changes in the strength of a magnetic field. The known method comprises inducing transient eddy currents in the object; receiving a signal indicative of the eddy current, and comparing the decay of the received signal over a period of time with a reference decay indicative of a known wall thickness, whereby the thickness of the container means wall portion can be inferred.
In the method according to the present invention, a probe is used, which comprises a transmitter coil for inducing eddy currents in the object, and a receiver system for providing a signal indicative of the strength of a magnetic field or changes in the strength of a magnetic field.
Examples of objects that can suitably be inspected with the method according to the present invention are metal plates or walls of container means, such as pipes, vessels or containers, having a radius of curvature that is larger than the thickness of the object. The electrically conducting material can be any electrically conducting material, for example carbon steel or stainless steel.
It is an object of the present invention to provide an improved method for inspecting an object so as to locate anomalies. It is a further object to provide an inspection method that is faster than the current inspection methods and which is less susceptible to variations in the electromagnetic properties of the object. A further object is to detect smaller anomalies than was possible with the known method.
To this end, the method of inspecting an electrically conductive object so as to detect the presence of an anomaly using a probe, which probe comprises a transmitter coil for inducing eddy currents in the object, and a receiver system for providing a signal indicative of the strength of a magnetic field or changes in the strength of a magnetic field, which receiver system includes at least one receiver, according to the present invention, comprises the steps of:
(a) selecting a set of points on the near surface of the object that are to be inspected;
(b) selecting a first inspection point from the set;
(c) positioning the probe at the selected inspection point, inducing transient eddy currents in the object and determining a characteristic value, "PHgr", that relates to the amplitude of the signal of the receiver;
(d) selecting a next inspection point from the set and repeating step (c) until all inspection points have had their turn; and
(e) inferring that an anomaly is present at an inspection point if the characteristic value "PHgr" differs significantly from a norm.
Suitably the probe contains at least two spaced apart receivers. Then step (c) comprises positioning the probe at the selected inspection point, inducing eddy currents in the object and determining a characteristic value, "PHgr", that relates to the gradient of the electromagnetic field.